This invention relates to the art of chemically treating an aluminum surface. More specifically, it relates to the art of treating an aluminum surface to improve both the corrosion resistance of a bare surface and the adhesion of an organic finish subsequently applied to the treated surface. This invention also concerns a composition and process for accomplishing the foregoing results without causing the waste disposal problems created by the use of compositions containing large concentrations of phosphate and/or hexavalent chromium compounds.
The use of tannins in connection with metal treating has been suggested by the prior art. U.S. Pat. No. 2,502,441 discloses an alkali metal phosphatizing solution containing a two-component accelerator which may be used for the treatment of iron and steel surfaces and also possibly for other metals such as aluminum. The accelerator portion of the composition contains either a molybdenum or tungsten compound and a phenolic substance such as a tannin. The patentee notes, however, that if the alkali metal phosphate plus tannin is used without the molybdenum compound, deposition of a coating seems to be completely inhibited. U.S. Pat. No. 2,854,368 teaches the use of a phosphoric acid solution containing a tannin for the treatment of iron or steel and also possibly for other metals such as aluminum. The most dilute solution suggested by the patentee is one containing one mole of phosphoric acid per liter and one weight percent tannin.